Stabilizing method and system utilizing nuclear precession



March 18, 1952 w. D. HERsHBl-:RGER 2,589,494

STABILIZING METHOD AND SYSTEM UTILIZING NUCLEAR PRECESSION Filed July20, 1948 3 Sheets-Sheet l March 18, i952 w D. HERSHBERGER 2,589,494

STAB.. ZING METHOD AND SYSTEM UTILI ING NUCLEAR PRECESSION Filed Ju1y-2o., 1948 .s Sheets-sheet 2 waan/7k 4 March 18, 1952 w. D. HERSHBERGER2,589,494

STABILIZING METHOD-AND SYSTEM UTILIZING NUCLEAR PRECESSION Filed July20, 1948 3 Sheets-Sheet 5 wwwa/Yage@ .By

Patented Mar. 18, 1952 STABILIZING METHOD AND SYSTEM UTILIZING NUCLEAR-PRECESSION William D. Hershberger, Princeton, N. J., assigner to RadioCorporation of America, a corporation of Delaware Application July 20,1948, Serial No. 39,792

29 Claims. (CI. Z50- 36) This `'invention relates to methods and systemsutilizing nuclear procession for stabilization of magnetic fields.

In accordance with the invention, the intensity of a unidirectionalfield, or the frequency of an alternating field, may be stabilized byapplying both fields to any selected nuclear reaction source and byvarying the intensity of the uni` directional field, or the frequency ofthe alternatingfield,1to maintain nuclear procession which l occurs onlywhen the ratio of the intensity .of the unidirectional eld to thefrequency of the alternating field closely corresponds with a constantwhich is characteristic of Jthe particular selected nucleus.

More specifically, the unidirectional and alterhating magnetic fieldsapplied to the nuclear reaction source are produced, in some forms ofthe invention, by coils which are disposed mutually at right angles toeach other and to a third pickup coil effectively magnetically coupledto them only during nuclear procession to provide a signal usable forstabilization: in other forms of the invention, voltage, current, fieldor other effects of the unidirectional and alternating fields arecombined in a bridge circuit whose alternatingcurrent imbalance,occurring upon change in nuclear procession, provides the signal usedfor stabilization.

In some forms of the invention, the output of receiver apparatussupplied from the abovementioned tertiary coil is used to control thefrequency of a radio-frequency generator which suppliesthe alternatingfield coil: in other modifications, the tertiary coil and thealternating field collare included in the feedback. loop or" aself-excited oscillator operating at the precession frequency.

- In modifications used for stabilization of the unidirectional field,the variations of the difference between the precession frequency and astandard frequency are used to control a variable componentrof the totalunidirectional eld; in some modifications, the intensity of theunidirectional field isstabilized by adjusting a Variable componentthereof to maintain a fixed difference between the frequency of thegenerator which supplies the alternating field and the frequency of asecond generator stabilized by a precision standard, such as apiezo-electric quartz crystal, ora molecular resonance absorptionspectral line of a. "microwave absorptive gas; in other modifications.the variable component of the unidirec` tionaliieldls adjusted tomaintain a constant phase d'ncrenc'e between two trains of pulses,

one produced by combining the outputs of the tertiary coil and a sweeposcillator and the other train produced by combining the outputs of thesweep oscillator and the stabilized frequency source.

The invention further resides in methods and systems having the featuresof novelty hereinafter described and claimed.

For a more detailed understanding ofthe in vention and for illustrationof various embodiments thereof, reference is made to the accompanyingdrawings in which:

Figures 1 and 2 are diagrams of frequencystabilization systems utilizingnuclear precession;

Figures 3 and 4 are diagrams of intensitystabilizaticn systems using thetri-coil arrangement of Figures l and 2; and

Figures 5 and 6 are diagrams of intensitystabilization systems using abridge instead of the tri-coil arrangement of the preceding figures.

From investigations in nuclear physics, it has been established that themagnetic moment (u) of a nuclear reaction source may be expressed aswhere f=frequency of procession h.=Plancks constant I=spin numberH=magnetic field intensity Of the five terms in Equation l, three ofthemthe magnetic moment, Plancks constant, and the spin number-areabsolutely fixed for any given substance and therefore there is suchdirect or rigid relationship between the magnetic field intensity andthe frequency of p-recession that either the frequency or the fieldintensity may be used as a standard for stabilization of the other.Nuclear procession (proton resonance) occurs .when and only when theratio between the ntensity of the unidirectional eld and the frequencyof the alternating field exactly or very closely corresponds with anumerical constant which is characteristic of the nucleus selected as areaction source. Although the foregoing is applicable to any nucleuswhich has spin and a magnetic moment, the subsequent discussions Will belimited for clarity to protons; i. e., hydrogen nuclei whose spin numberis 1/2. whose nuclear angular momentum is 3 and Whose magnetic moment is2.79 nuclear magnetons. (For completeness here, it may be stated that eh (2) 1 nuclear magnetonf-11T;Z

where I-/=ratio of charge to mass of the proton c=speed of light-:SXlOwcnr/second) Referring to Figure 1 as exemplary of a frequency-controlsystem utilizing nuclear precession, the coils I are supplied withdirect current from a stabilized source, generically represented by thebattery II and rheostat I2, to produce a magnetic field Het whichtraverses a nuclear reaction source, for example, Water, whose nucleihave spin and a magnetic moment. The water may be disposed in a smalltest-tube, ampule or other non-magnetic container. The coil I3 issupplied with current from a radio-frequency generator Ill to produce analternating magnetic field HT at right angles to the unidirectionalfield Hdc. The axis of the tertiary pickup coil Iis at right angles tothe axes of both the coils I0 and I3, so that, except for nuclear-precession, it is not coupled either to the unidirectionalfield or tothe alternating eld. Thus, in* effect the axes of coils I0, I3 and I5dene a tridimensional coordinate system Whose three, mutually-normalplanes pass through the nuclear reaction source located at the origin orintersection point of the planes.

The desired operating frequency of generator I4 corresponds with theprecession frequency or rate of the particular substance chosen as anuclear standard at the selected intensity of the unidirectional fieldHee: conversely stated, for any chosen nuclear standard theunidirectional field intensity is selected or adjusted to eect nuclearprecession at the desired operating frequency of generator I4, or at afrequency derived therefrom for the excitation of coil I3. For ex- Lample, assuming as above, that the chosen standard is the hydrogennucleus and that the selected intensity of the unidirectional eld Het is2,000 gauss, the frequency of precession is very close to 9 megacycles.A lithium nucleus, in this frequency'Within a Wide band of thefrequency` spectrum.

.At and close vto the precession frequency, the

pickup coil I5'i`s effectively coupled to the magnetic` fieldstraversing the nuclear standard `and accordingly supplies an inputvoltage to the radio receiver IS which, utilizing the well-knowndiscriminator-and reactance-tube combination, for example, may vbe usedto stabilize the frequency of thegenerator I4 so that it may be used asa primary standard of frequency either for test purposes or forexcitation of power amplifier stages tuned to the same frequency or toharmonically related frequencies. Under the foregoing conditions, at theprecession frequency of 9 mega- 1 cycles, a signal voltage of the orderof 5 millivolts was developed across a receiver coil I5 having a Q ofabout 80 when the'intensity of the A. C.

' the oscillator I4 is stabilized to operate on either steep slope of anuclear (proton) resonance curve affording large frequency-controleffects for minute deviations of frequency.

In the system shown in Figure 2, the pickup coil I5 is included in theinput circuit of the high-gain amplifier generically represented by tubeI'I and block I8. The operating grid-bias of tube I'I may be derivedfrom its rectied gridcurrent by the resistance-capacitance network I9,20. The D. C. anode-current supply source is. generically represented bybattery 2|. The .coil

I3 for supplying the A. C. magnetic field impressed on the nuclearreaction source is includeA ed in the output circuit of the amplifier I8and is" so poledor phased there is formed a self-excited' oscillatorsystem for generation of oscillations at the precession frequency.Feedback coupling between coils I3, I5 exists only at andvery'close totheprecession frequency. `The feedback loop of the oscillator preferablyincludes gain and phase-controls generically represented by knobs I8G,ISP. In other respects, this modification is` similar to that of Figure1 and need not be further discussed.

field Hdc is in part supplied by the coils I0 enerfgized from a fairlystable or stabilized Isource II v and in part supplied by the coil 25which provides the variable component Hdcx of the total unidirectionalmagnetic eld applied to the nuclear standard. The variable field Hm mayoppose orv aid the field of coilsY I 0: rin either event, the total'unidirectional iieldis the algebraic sum of the' elds of coils I0 and'25.

In the particular arrangement shown in Fig;i

ure 3, the auxiliary field coil 25 is in the anode circuit of a controltube 26 for traverse by the direct-current component of the anodecurrent of the tubeas supplied-by a-direct-current source exemplified bybattery 21. The alternatingcurrent component of the anode current is ex#eluded from coil 25 as by a filter or by-pass arrangement represented bythe capacitor 29. Thef excitation for coil I3 Which supplies thealter-`- nating magnetic field for the nuclear standard may be providedas in Figures 1 and 4 by an osfrequency f1, derived from a stabilzedfrequency,

generator 3I. For many purposes, the frequency of generator 3Imay bestabilized Within satisfactorily close limits by a piezo-electriccrystal.

'Ihe difference frequency (fof-f1) or (freie) appearing in the output ofmixer au isappued' Y to afrequency-counter circuit 34 of anyvsuitable',v type, such for. example, 'as shown'in Sanders.

Patent No. 2,228,367, and the direct-current outputof thefrequency-counter circuit is supplied variably to bias the grid of thefield-control tube 26. Accordingly, as the total intensity of theunidirectional field jointly produced by coils l and tends to deviate ineither direction from the desired value. the precession frequency fccorrespondingly varies: in consequence, the differencefrequencyincreases or decreases in corresponding sense to change the grid voltageof the control tube 26 in the proper sense to effect acompensatorychange of field Haar.

- In brief, resonant frequency of the quartz crystal fr is used` as astandard of reference for the precession frequency and theunidirectional magnetic field Het is controlled to maintain a constantdifference between the quartz crystal frequency and the nuclear resonantfrequency of the selected nuclear standard. As the resonant frequency ofthe quartz crystal is extremely sharp and since the precession frequencyof the nuclear reaction source is rigidly related to the intensity ofthe field applied to it, the regulation of the field intensity inaccordance with deviations of the difference between those twofrequencies insures a constancy of the magnetic field intensity withinnarrow limits never heretofore approached and which are notsubstantially affected bytemperature, pressure or other ambientvariables.

In the modification shown in Figure 4, which is also for precisestabilization of a unidirectional field, the precession frequency fo ofthe nuclear reaction source and the periodically varying frequency of asweep oscillator 4i) are impressed upon a mixer or modulator 4l toproduce a beatfrequency applied to a Shaper-amplifier 42 which may be ofthe type shown in copending application Serial No. 4,497,flled January27, 1948, thus to generate two trains ofV pulses "a and b whoserepetition rate corresponds with the modulating frequency applied to thesweep oscillator 40. The waveform of the sweep or modulating frequencyis preferably sawtoothed, as indicated by pulses c though it may be ofother shape. The repetition rate of the sweep pulses may be low. forexample, of the order of 60 pulses per second and in any event lowrelative to the carrier frequency ofv oscillator 49. The output of thefrequency-modulated oscillator is also impressed upon a second'mixer 43for production of a varying beat-frequency equal to the differencebetween the modulated frequency of oscillator 40 and a.V fixed frequencyfr derived from a stable or stabilized oscillator 44 whosefrequency-determining standard may be a piezo-electric crystal or amicrowave absorptive moecular resonant gas cell. The frequency frprecisely corresponds with or bears a fixed numerical relation to theprecession frequency of the nuclear reaction source at the desiredintensity of the magnetic field Hee, depending upon the chosenfrequency-selective,characteristic of the shaperamplifiers 42 and 45.If, for example, the frequency-selective networks of amplifiers 42, 45are low-pass filters, the desired frequency difference i-s zero and theband of frequencies swept by oscillator 40 includes the frequencies foand f1.

The output of the second. rriXer 43 is impressed upon theShaper-amplifier 45, which may be of the type shown in copendingapplication Serial No. 4,497 to generate a sawtooth wave whoserepetition frequency corresponds with the modulating frequency appliedto the sweep oscillator 42 and which is used as a phase reference. Thesawtoothv waves from network 45 andthe sharp pulses from network 42 areapplied to phase detector 46, which may be of the type shown-in WendtPatent No. 2,250,284, to produce a unidirectional bias voltage for thefield-control tube 26.

Accordingly, when the intensity of the magnetic field Het deviates fromits desired value, there is a corresponding change in the precessionfrequency fo with resultant shift in phase of the pulses a, b withrespect to the sawtooth im.- pulses c. In consequence, the control-gridbias of tube 2B automatically increases or decreases in sense varyingthe excitation of the auxiliary field coil 25 to restore the total fieldintensity to the desired proper magnitude.

Instead of using the geometrical tri-coil arrangement of Figures 1 to 4for stabilization of frequency or magnetic-field intensity by a nuclearreaction source, there may be used the bridge arrangements of Figures 5and 6. As in the previously described modifications, the nuclearreaction source is disposed in a unidirectional magnetic eld Hdc(produced by coils I0 for example) and in an alternating magnetic fieldHr at right angles or normal to t-he unidirectional field. The inputcircuit of receiver I6 for detecting and responding to variations of thenuclear precession signal is connected in one arm of the bridge, Figure5, between output terminals 50, 5l thereof: the output circuit of theamplifier or oscillator ifi, i8 is effectively connected in a conjugatearm of the bridge between terminals 52, 53. In the particulararrangement shown, the output of amplier or oscillator i4, i8 is coupledto the input arm of the bridge by transformer 53 whose secondary windingis center-tapped for connection to output terminal 5l of the bridge.

With the nuclear reaction source out of the field both bridge coils 54A,54B, the bridge is balanced, as by adjust-ment of one or lboth of thecondensers 55A, 55B at the frequency ,fo correcn source at the desiredintensity of field With the bridge so balanced, there is no reciablecoupling between the input circuit or the receiver le and the outputcircuit of .the oscillator or amplifier h, 3. The reaction source isthen inserted in one of the bridge coils, as shown, Figure 5, toestablish a coupling for generation or stabilization of oscillations atthe precession frequency. As in the system of Figure 3, any variation cfthe difference between the precession frequency fo and a standardfrequency fr is utilized to control the magnitude of directcurrenttraversing the auxiliary direct-current eld-coil 525. The sense andextent of the variation of current in coil 25 restores the totalintensity of the unidirectional field to the desired magnitude.

The arrangement shown in Figure 5, as well as those of precedingfigures, may be used for stabiliz-ation of the field intensity ofelectromagnets used in cyclotrons, betatrons, mass spectrometers and thelike.

in Figure 6, the bridge network 52A is a variant of the `bridge 52 ofFigure 5. The primary 5l of transformer 53A is differentially coupled toits secondary windings 56A, EB which are respectively connected to thetuned circuits 54A, 55A and 54B, 55B isolated from each other as bydisposition in different shielded compartments. The bridge is balancedin two operations: for amplitude, by varying the coupling of the primary51 to the secondary windings 56A and 56B; and for phase, by adjustmentof the tuning condensers 55A, 55B or equivalent to tune their respectivecircuits to exact resonance at the frequency of the signal generator I4or equivalent. The pickup coils IEA, ISB respectively coupled to thebridge coils 54A, 54B are connected in opposition in the input ofreceiver I6. Therefore, at balance of :bridge 52A, no signal isimpressed upon the receiver. After the'bridge is so preliminarilybalanced, the nuclear reaction source is inserted in a selected one ofthe coils 54A, MB so that as in the preceding modifications the reactionsource is subjected both to an alternating field and to a relativelystrong unidirectional field normal thereto.H At and very close to theprecession frequency of the reaction source, the reactance and the Q ofthe directly associated tuned circuit MA, 55A is markedly changed sounbalancing the bridge and causing a signal to appear in the receiverI6. Also as in the preceding modifications, this unbalance signal may beused for stablization of the unidirectional eld or of the frequency ofthe alternating eld.

As pointed out heretofore, the inventiony is not limited to the choiceof the hydrogen-nucleus o1' proton, but 'is equally well adapted to theuse of many oher nuclei whose moments have been determined. Twenty-sixsuch nuclei are listed in the' Reviews of Modern Physics, volume 18, No.3, page' ,may be made within the scope of the appended claims.

I claim as my invention:

1. The method of stabilization which comprises Y applying aunidirectional field and an alternating field to a nuclear reactionsource, and maintaining a constant ratio of the intensity ofsaid'unidirectional field to the frequency of said alternating field toeffect continuous nuclear precession.

2. The method which comprises applying a unidirectional field and analternating field to a nuclear reaction source to effect nuclearprecession, producing a control effect varying in accordance with theprecession frequency, and applying said control effect to maintainconstant the ratio of the'intensity of said unidirectional field to thefrequency of said alternating field.

3. The method of stabilizing the intensity of a unidirectional eld whichcomprises applying said field and arelatively weak alternating field toa nuclear reaction source to effect nuclear precession, producing acontrol effect varying in accordance with deviation of the precessionfrequency from a standard frequency,V and applying saidV control effectto regulate the intensity of said unidirectional field.

4. The method of employing nuclear precession ln a magnetic field forstabilizing the magnetic eld intensity comprising establishing a radiofrequency field at the nuclear precession rate for a predetermined fieldintensity, detecting radio frequency energy coupled through saidmagnetic field substantially only by nuclear precession, and utilizingsaid detected energy to stabilize the intensity of said magnetic field.

5. The method of employing nuclear precession in a magnetic field forstabilizing the magneticv field intensity comprising establishing aunidirectional magnetic field, establishing a'radio frequency eld at thenuclear precession rate for a predetermined field intensity, detectingradio frequency energy coupled through said magnetic field substantiallyonly by nuclear precession. and establishing a compensatingunidirectional magnetic field in response to said detected energy tostabilize the intensity of said magnetic field.

6. The method of employing nuclear precession in a magnetic field forstabilizing the magnetic field intensity comprising establishing aunidirectional magnetic field, establishing a 4radio frequency fieldnormal to said unidirectional -eld at the nuclear precession rate for apredetermined field intensity, detecting radio frequency energy coupledthrough' said magnetic field substantially only by nuclear precession ina plane normal to the plane of said radio frequency field, andestablishing a compensating Vunidirectional magnetic` field in responseto said detected energy to sta-4' bilize the intensity of said magneticeld. j

7. The method of employing `nuclear precession in a magnetic eld forstabilizing the magnetic field intensity comprising establishingl aunidirectional magnetic eld, establishing a radio frequency field normalto said unidirectional field at the nuclear precession rate' fora'predetermined field intensity, detecting radio frequency energycoupled through said magnetic field substantially only by nuclearprecession in a plane normalto y the plane of said radio frequency fieldto derive a rst signal at the frequency of said radio frequency field,generating a second signal of a`v different frequency, mixing saidsignals to pro--r` vide a'dierence frequency signal, deriving av con'-trol signal characteristic of the frequency of said difference frequencysignal, and applying saidy control signal to stabilize the intensity ofsaid magnetic nele. 8. The method according'to claim 7 includingapplying said control signal to establish a compensating unidirectionalmagnetic field to *stabilize the total intensity'of said magnetic field.9. The method of employing nuclear precession in a constant intensitymagnetic field forstabilizing the frequency of a radio frequency sourceat the nuclear precession rate comprising estab# lishing avunidirectional magnetic field,`coupling said radio frequency source tosaid field, detecting radio frequency energy coupled through said fieldsubstantially only by nuclear precession, andl utilizing said detectedenergy to stabilize the frequency of said source at thenuclear`precessin rate.

l0. The method of"employingnuclearvprecession in a constant intensitymagnetic field for stabilizing the frequency of a radio frequency sourceat the nuclear precession rate comprising establishing a unidirectionalmagnetic field, coupling s aid radio frequency source to said field,detecting radio frequency energy coupled through said fieldsubstantially only by nuclear precession in a plane substantially normalto the plane of said coupled energy, and coupling said detected energyto said source to stabilizethe frequency of said source at the nuclearprecession rate. 11. The method of employing nuclear precession in aconstant intensity magneticfield forvl controlling the generation ofradio frequency energy at the nuclear precession rate comprisingestablishing a unidirectional `magnetic field, gen,-4 erating radiofrequency energy,` "coupling ,saidV radio frequency energy to saidfield, deriving radio frequency energy coupled through said field-sub-1'2. The method of employing nuclear precession in a magnetic fieldwherein the density of said field is directly related to the frequencyof radio frequency energy propagated through said field comprisingestablishing a unidirectional magnetic eld, establishing in saidunidirectional magnetic field a radio frequency field at the nuclearprecession rate, stabilizing the aforementioned characteristic of one ofsaid fields and controlling the aforementioned characteristic of theother of said fields in response to said iield'stabilization.

Y' 13. A system employing nuclear precession in a ,magnetic fieldwherein the density of said field is directly related to the frequencyof radio frequency'energy propagated through said field comprising meansfor establishing a unidirectional magnetic field, means for establishingin said unidirectional magnetic field a radio frequency fieldat thenuclear precession rate, and means for stabilizing the aforementionedcharacteristic of one of said elds to control the aforementionedcharacteristic of the other of said fields.

14. Apparatus for employingvnuclear precession in a magnetic field forstabilizing the magnetic field intensity comprising means forestablishing a radio frequency field at the nuclear precession rate fora predetermined field intensity, means for detecting radio frequencyenergy coupled through said magnetic eld substantially only by nuclearprecession, and means for utilizing said detected energy to stabilizethe intensity of said magnetic field.

15. Apparatus for employing nuclear precession in a magnetic field forstabilizing the magnetic field intensity comprising means forestablishing a radio frequency field at the nuclear precession rate fora predetermined field intensity, means for det .cting radio frequencyenergy coupled through said magnetic field substantially only by nuclearprecession in a plane normal to the plane of said established radiofrequency field," means for deriving from said detected energy `acontrol signal, and means responsive to said control signal forestablishing a compensating unidirectional field to stabilize theintensity of said magnetic field.

Q16. Apparatus for employing nuclear precession v'in a magnetic fieldfor stabilizing the magnetic field intensity comprising means forestablishing a unidirectional magnetic field, means for establishing aradio frequency field at the nuclear precession rate for a predeterminedmagnetic field intensity, means for detecting radio frequency energycoupled through said magnetic field substantially only by nuclearprecession in a plane normal to the plane of said established radiofrequency field, means for deriving from said detected energy firstsignals of frequency dependent upon said precession rate, meansproviding second signals of different frequency than said first signals,.means for combining said rst and second signals to derive differencefrequency signals, means responsive to said difference frequency signalsfor deriving control signals, and means responsive to said controlsignals for establishing a compensating unidirectional magnetic field tostabilize the total intensity of said magnetic field.

netic field intensity comprising means for establishing a unidirectionalmagnetic field, means for establishing a radio frequency fleld at thenuclear precession rate for a predetermined magnetic field intensity.means for detecting radio fre- Aquency energy coupled through saidmagnetic va plane normal to the plane of said established radiofrequency field, means for deriving from said detected energy firstsignals of frequency dependent upon said precession rate, meansproviding Vsecond signals of different frequency than said firstsignals, means for combining said first and second signals to derivedifference frequency signals, frequency counter circuit means responsiveto said difference frequency signals for deriving control signals, andmeans responsive to said control signals for establishing a compensatingunidirectional magnetic field to stabilize the total intensity of saidmagnetic field.

v18. Apparatus for employing nuclear precession in a magnetic field forstabilizing the magnetic eld intensity comprising means for establishinga unidirectional magnetic field, means for establishing a radiofrequency field at the nuclear precession rate for a predeterminedmagnetic field intensity, means for detecting radio frequency energycoupled through said magnetic field substantially only by nuclearprecession in a plane normal to the plane of said established radiofrequency field, means for deriving from said detected energy firstsignals of frequency dependent upon said precession rate, meansproviding second signals of different frequency than said first signals,means for combining said rst and second signals to derive differencefrequency signals, frequency counter circuit means responsive to saiddifference frequency signals for deriving control signals, andthermionic tube means responsive to said control signals and coupled tosaid magnetic field for establishing a compensating unidirectionalmagnetic field to stabilize the' bilize the total intensity of saidmagnetic field.

20. Apparatus according to claim 19 wherein said radio frequency fieldestablishing means is stabilized in frequency with respect to amolecular resonance absorption spectral line cf a micro- Wave absorptivegas.

21. A radio frequency oscillator employing nuclear precession in asubstantially constant intensity magnetic field for controlling the fre-17. Apparatusfor employing nuclear preces-v sion in a magnetic field forstabilizing the vcomprising means for establishing a unidirectionalmagntic field, a radio frequency energy 1l source coupled to said field,means for detecting radio frequency energy coupled through said fieldsubstantially only by nuclear precession, and means for coupling saiddetected energy regeneratively to said source to stabilize the frequencyof said source at the nuclear precession rate.

23. A generator employing nuclear precession in a constant intensitymagnetic field for controlling the frequency of the generated radiofrequency energy at the nuclear precession rate comprising means forestablishing a unidirectional magnetic field, an input circuit and anoutput circuit coupled to said field, said circuits being coupledtogether through said field substantially only by nuclear precession,and a thermionic amplifier coupled to said circuits for regenerativelycoupling said circuits tol provide sustained radio frequencyoscillations at the nuclear precession rate.

24. The method of employing nuclearv precession in a magnetic field forstabilizing the magnetic field intensity comprising establishing a radiofrequency field at the nuclear precession rate, establishing other radiofrequency energy of slightly .different frequency than said rate,detecting radio frequency energy coupled through said fieldsubstantially only by nuclear precession, detecting said other energy,modulating both of said detected energies, comparing said modulateddetected energy, and utilizing the difierence between said comparedenergies to stabilize the intensity of said magnetic eld.

25. Apparatus for employing nuclear precession in a magnetic fieldfornstabilizing the magnetic. field intensity comprising means forestablishing a unidirectional magnetic field, means for establishing insaid magnetic field a radio frequency field at the nuclear precessionrate, means for deriving first signals from said radio frequency fielddue substantially only to coupling therethrough by'nuclear precession, asource of timing signals, a first signa] mixer network means forapplying said first signals and said timing signals to said firstnetwork, a source of second radio frequency signals of frequencyslightly differing from said first signal frequency, a second signalmixer network, means for applying said second signals and said timingsignals to said second network, a phase comparison circuit, means forapplying mixed signals from said mixer networks to said phase comparisoncircuittoderive a control signal of magnitude-dependy ent upon the phaserelations of said mixed signals and means for applying said controlsignal 12 for establishing in said magnetic field a radio fre quencyfield at the nuclear precession rate, a second source cf radio frequencysignals of frequency slightly differing from said precession rate, meansfor sweeping said first radio frequency signals through said precessionfrequency and through the second radio frequency means for comparing thephase of said frequency swept signals to derive a control signal ofmagnitude dependent upon said phase relation, and means for applyingsaid control signal to stabilize the intensity of said unidirectionalmagnetic field. l

27. The method of employing nuclear precession in a magnetic field forstabilizing thermagnetic field intensity comprising establishing aunidirectional magnetic field, establishing in said magnetic field aradio frequency field at the nuclear precession rate, a balanced networksubjected to said fields and arranged to be unbalanced by nuclearprecession in one portion thereof due to increase in field intensityabove a predetermined value, means for detecting currents due to saidunbalancingof said network, means responsive to said detected currentsvfor deriving control currents, and means for lapplying said controlcurrents to stabilize the intensity of said unidirectional magneticfield. Y

28. The method of employing nuclear precession in a magnetic field forstabilizing themagT netic eld intensity comprising establishing aunidirectional magnetic field, establishing in said magnetic eld a radiofrequency field at the nuclear precession rate, generating radiofrequency signals of frequency slightly differing from said precessionrate, sweeping said Vfirst radio frequency signals through said-precession frequency and through said second radio frequency, comparingthe phase of` said frequency swept signals to derive a control signal ofmagnitude dependent upon said phase relation, and applying said controlsignal to stabilize the intensity of said unidirectional magnetic field.

29. Apparatus for employingnuclear precession in a magnetic field forstabilizing the .mag-

netic field intensity comprising means for establishing a unidirectionalmagnetic field, means for establishing in said magnetic field a radiofrequency field at the nuclear Yprecession rate, a balanced networksubjected to said fields and arranged to be unbalanced by nuclearprecession lin one portion thereof due to increase in field intensityabove a predetermined value, meansfor detecting currents due to saidunbalancingn ofsaid network, means responsive to said detected 4currents for deriving control currents, and means --for applying saidcontrol currents to stabilizeV No references cited.

